Handle default values of nested elements

Tested with version 1.0.0-beta5. Worked great for me, thanks for the solution.

So, patch #5 have fixed some occurrences in one of our sites but in other forms it had the exact opposite effect, causing the bug to occur where once it was working just fine.

I tweaked patch #5, it worked for us. ( tested with 1.0.0-beta4 ).

Damn... can someone please rename the patch above to reflect the comment ?

#9 tested with version 1.0.0-beta6. Work for me.

In buildForm() I have:

$form['legal_links'] = [
      '#type' => 'multivalue',
      '#title' => $this->t('Legal links'),
      '#description' => $this->t('Links to legal documents in the enrollment form. Leave empty a row to remove it.'),
      '#cardinality' => MultiValue::CARDINALITY_UNLIMITED,
      '#default_value' => $links,
      'link_title' => [
        '#type' => 'textfield',
        '#title' => $this->t('Title'),
        '#description' => $this->t('To include a link, please provide a title as well.'),
      ],
      'link' => [
        '#type' => 'textfield',
        '#title' => $this->t('Link'),
        '#maxlength' => 2048,
        '#element_validate' => [[static::class, 'validateUriElement']],
        '#link_type' => LinkItemInterface::LINK_GENERIC,
        '#description' => $this->t(
          'You can enter an internal path such as %add-node or an external URL such as %url. Enter %front to link to the front page. Enter %nolink to display link text only. Enter %button to display keyboard-accessible link text only.',
          [
            '%front' => '<front>',
            '%add-node' => '/node/add',
            '%url' => 'http://example.com',
            '%nolink' => '<nolink>',
            '%button' => '<button>',
          ]
        ),
      ],
    ];

And to save, in submitForm():

$this->config('mymodule.settings')
      ->set('legal_links', $form_state->getValue('legal_links'))
      ->save();

Test coverage has to be added.
Off the top of my head, I think I didn't add support for nested elements because of the possibility of having #tree set to true or false, making it hard to handle complex cases. Please test that scenario when adding tests.

text_format elements added along this.

Setting to NW as maintainer has asked for tests.